Identify your use: roasting/caramelizing (need sweetness and tenderness) or raw/as-a-garnish (need mild bite).
Choose the closest swap from the table: Vidalia or Walla Walla for the mild, sweet profile.
Use the listed ratio; if your substitute is sharper (yellow or red), slice thinner and cook longer to mellow the bite.
For watery swaps (red onion), blanch briefly and drain before adding to avoid steaming.
Taste once cooked: add a tiny pinch of sugar or salt to steer toward cipollini’s sweet-savory balance.